Progressive multifocal leukoencephalopathy (PML) is a fatal demyelinating disease caused by human neurotropic JC virus. JCV infects more than 80% of human population during the childhood and establishes a latent infection for the rest of the life. The virus reactivates under immunomodulatory conditions with an unknown mechanism, resulting in productive infection of oligodendrocytes and astrocytes in the central nervous system (CNS). JCV replicates almost exclusively in glial cells, and its promoter sequence, which has tissue- specific characteristics, tightly modulates expression of viral genome in appropriate cell types and immune conditions through communication with cellular factors. We recently identified the alternative splicing factor, SF2/ASF, as a potential regulator of JCV as its overexpression in glial cells strongly suppresses viral gene expression and replication. Unexpectedly, down-regulation of JCV by SF2/ASF is mediated at transcriptional stage, thus ascribing a novel role for SF2/ASF in the control of promoter activity. Results from our preliminary studies suggest that immune mediators secreted by PBMCs induce the expression of SF2/ASF, and inhibit the replication of JCV. These observations suggest operation of a novel immune signaling pathway between peripheral immune cells and glial cells that controls the immediate early stage of JCV gene expression during the course of viral reactivation. Our preliminary studies from JCV-infected glial cells also revealed a significant role of agnoprotein, a small viral protein crucial for JCV replication, in regulation of SF2/ASF. Expression of SF2/ASF is induced, and colocalized with agnoprotein in the cytoplasm of infected cells. Results from biochemical assays showed interaction of agnoprotein with SF2/ASF suggesting a functional cross-association between these two proteins. Results from reporter gene assays revealed that agnoprotein induces the viral transcription silenced by SF2/ASF. These observations provided us a rationale to study the role of SF2/ASF in JCV life cycle, and led us to formulate our central hypothesis which s that neuroimmune regulation of JCV replication is mediated by SF2/ASF, and viral agnoprotein interferes with this regulation. We propose to examine our hypothesis by (i) examining immune mediated regulation of SF2/ASF and replication of JCV during the latent as well as productive period of the viral infection and (ii) investigating the functional interplay between SF2/ASF and JCV agnoprotein in this regulation. We will employ molecular, cellular, and virological approaches to address these questions and examine the biological relevance of our findings by immunohistochemical evaluation of clinical samples from patients with PML disease. The information from these studies will offer a novel strategy for the treatment of PML.